I. Technical Field
The present invention relates to an electromagnetic reciprocating fluid device, such as a pump and a compressor, which reciprocates a piston by generating an alternating magnetic field by means of electromagnets.
II. Description of the Related Art
Such electromagnetic reciprocating fluid devices are generally divided into two types. In the first type, a magnetically attractable portion made of a magnetic material is disposed on a movable member (a piston rod) connected to a piston and the movable member is reciprocated by an electromagnetic attraction force periodically generated by electromagnets to act on the magnetically attractable portion and a spring force biasing the movable member in a direction opposite to the direction in which the magnetically attractable portion is attracted by the electromagnetic attraction force. In the second type, the movable member is equipped with a permanent magnet, and the permanent magnet is attracted and repelled by an alternating magnetic field generated by the electromagnets, whereby the movable member is reciprocated.
The movable member is disposed, with a slight clearance, between electromagnets oppositely arranged on both sides across the reciprocating path. The slight clearance needs to be maintained. Therefore, in the first type described above, for example, both ends of the movable member are slidably held to maintain the clearance (Japanese Examined Patent Application Publication No. 57-30984).
In the second type described above, the movable member has a permanent magnet. Therefore, in addition to an attracting force and a repelling force in the reciprocating direction, a transverse force with respect to the reciprocating direction acts on the movable member. Thus, torque about the longitudinal axis of the movable member might be generated. Generally, the permanent magnet attached to the movable member has a square cross section. Therefore, if the permanent magnet is inclined about the longitudinal axis by torque, an attracting force by one electromagnet becomes larger than that by the other electromagnet, which might cause friction of the permanent magnet against the one electromagnet. In general, each end of the movable member is designed to be held by means of a piston having a circular cross section and a cylinder slidably encasing the piston. Therefore, the inclination of the movable member about the longitudinal axis cannot be prevented. In order to avoid this inclination, a device in which each end of the movable member is supported by a diaphragm or a cross-shaped support spring having flexibility in the reciprocating direction and stiffness about the axis is disclosed (Japanese Unexamined Patent Application Publication No. 2002-213354). However, such support members restrict the motion of the movable member in the longitudinal direction, which is not preferable for a fluid device.